make kv_f16 the default for api users (ggml-org#1517)

Author: Green-Sky

llama.cpp

@@ -822,7 +822,7 @@ struct llama_context_params llama_context_default_params() {
         /*.n_ctx                       =*/ 512,
         /*.gpu_layers                  =*/ 0,
         /*.seed                        =*/ -1,
-        /*.f16_kv                      =*/ false,
+        /*.f16_kv                      =*/ true,
         /*.logits_all                  =*/ false,
         /*.vocab_only                  =*/ false,
         /*.use_mmap                    =*/ true,